1. Field of the Invention
This invention relates to a hydraulic jack for use in simultaneously stressing a plurality of elongate elements e.g. concrete reinforcing elements, or other steel cables which need precise tensioning, in the form of wire or strand
2. Description of the Prior Art
In recent years there have come into use hydraulic jacks which are capable of stressing simultaneously a large number of individual reinforcing elements, for instance as many as 150. The elements may each be for instance an individual wire or strand (i.e. a plurality of individual wires twisted together) or a group of wires not twisted together. Usually, such jacks have a gripping plate having a bore for each element which provides a conical seat for split conical gripping wedges which grip the element while the gripping plate is moved rearwardly, to stress the elements simultaneously.
After stressing, the elements are anchored in an anchoring plate which abuts the end concrete structure or is a part of the mould. Before stressing, split conical anchoring wedges are arranged in the conical seats in the bores in the anchoring plate through which the ends of the elements project. Conventionally, the type of jack described above has a lock-off piston (also called an anchoring piston) which is actuated after stressing to cause the anchoring wedges to be urged into their seats to grip the elements. It is usual to provide a lock-off plate which is driven forward by the lock-off piston so as to engage and drive home the anchoring wedges. The lock-off plate can be made removable from the jack, and it can then be used as a template to assist in the insertion of the elements into the jack before stressing.
After the anchoring wedges are secure, the jack must be released from the elements. One way of doing this is to provide a plate which is located in front of the gripping plate and has bores through which the elements pass and sleeves which surround the elements and extend from these bores into the bores in the gripping plate. Movement of the unlocking plates towards the gripping plate causes the sleeves to engage the gripping wedges to cause unlocking of the gripping wedges. The jack can then be removed from the elements. In known jacks, this unlocking action of the unlocking plate is performed only when the jack is closed up (i.e. the gripping plate is moved again to the front of the jack), following a stressing operation. In effect, in this closing up, since the gripping wedges are still secured to the elements, the body of the jack moves backwards away from the anchoring plate, and eventually an abutment in the front end of the jack strikes the unlocking plate to cause unlocking.
Examples of known jacks are illustrated in GB No. 1,402,655 and GB No. 1,527,601.
In many prior art jacks the gripping plate is located close to the front of the jack when it is closed up, and the plate is removable through the front of the jack (the gripping and unlocking plates must have an arrangement of apertures corresponding to the arrangement of elements, and so need to be changed to suit different arrangements).
However, it has been proposed in UK No. 1,449,320 to locate the gripping plate at the back of the jack. This proposed construction includes a movable part which acts as the lock-off plate. This part is not removed to act as a template in the insertion of the elements into the jack, but rather the jack is threaded with this part in situ. In order to guide the elements so that they approach the clamping plate properly located, this movable part is in the form of a cage or frame through which elements are threaded. This part extends the entire length of the jack and may also act as the unclamping means, protrusions on its rear end effecting unclamping as the jack closes up.
It has now been realised that certain improvements in aspects of these jacks are possible.